1. Field of the Invention
The present invention relates generally to the field of extracorporeal blood treatment systems, and more specifically to those portions of such systems relating to pressure sensing and pressure surge protection.
2. Description of the Prior Art
Various types of extra-corporeal blood treatment systems for humans, or external artificial kidneys, are, and have for some time, been available for removing waste products and impurities from the blood of patients having diseased, damaged, or impaired kidneys.
In order to perform this life saving function in a satisfactory manner, the systems must be provided with delicate system pressure sensing elements or transducers and associated electric or pneumatic controls for the system blood pump and valves. These transducers sense variations in system blood pressure as blood is pumped through the cleansing apparatus and, in response thereto, regulate the pump and valves to prevent high or low pressures which might cause damage to the patients' internal blood circulating system or delicate internal organs.
However, when these transducers are connected directly to the blood treatment part of the system, sharp high pressure transients, caused, for example, by equipment malfunction, may damage the transducers to the extent that they cannot later accurately sense, and hence control, system pressure. This may occur even if the pressure transient were such that the patient undergoing treatment was not injured. And even if a patient were injured, it might not be readily apparent that the transducer (or transducers) was damaged, and hence control of system pressure for future patients could be impaired.
In addition, if the transducer is directly coupled to the blood treatment portion of the system, it may be a source of contamination to the blood being treated. Even though the various blood lines and purifying apparatus may be replaced or sterilized after each use, the transducer, which is a relatively permanent part of the system, may be contaminated and introduce particles of a previous patients blood into a next patients blood. Or, if the transducer is damaged by pressure transients, it may become a source of other types of contaminants, such as metal particles, air, or oil.
Even if the transducer is not damaged by sharp blood pressure transients it may not be sufficiently responsive to control the system pressure within a pressure range not injurious to the patient.
As a result of these and other problems, the pressure transducer should be effectively isolated from blood being pumped through the system and means should be provided to absorb pressure transients both to protect patients and the transducer from injury. Although in some instances diaphragm type apparatus have been employed to isolate the pressure transducers from other portions of the blood treatment apparatus through which flows a patient's blood, such diaphragms, whose action depends upon stretching, are generally unsatisfactory because they prove to eventually crack in use and have not sufficient membrane memory to return to their original state, nor do they provide a means of warning if the isolator is over pressurized, absorb air or other contaminants from the sensor portion into the blood being treated, and also because the response of the diaphragm to pressure fluctuations in the blood treatment portion is always relatively constant and cannot readily be made to vary in a predetermined manner, as may be desirable in some applications.